Any hp increase from boring would come most likely from increased compression (larger displacement w/same size comb. chamber) or the unshrouding of the valves by moving the cylinder wall away from the valve, but it probably would not be enough difference to bore for hp if it is not otherwise needed. I would only bore the amount needed to insure a proper cylinder shape.
------------------
67 Fastback - Arctic White Pearl paint
351W ,Trick Flow Aluminum Heads, Edelbrock TorkerII, Carter 750 CFM, Comp. Cam 477/510 219/233@.050, Performance Automatic C-4 Trans, 3.55 gears, Front Disc Brakes, 1-1/8" Fr. 3/4" rear sway bars.
My 351W Fastback